Behave if the same view gets a11y focus twice.

In some corner cases, ViewRootImpl#setAccessibilityFocus
can be called twice for the same view. Make sure we're
actually changing views before clearing accessibility focus
to avoid inconsistent state.

Bug: 29637593
Change-Id: I5dc6f05bdeaec55c352417bfca92c11bd03c3c10
(cherry picked from commit da469277)